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[2003:cb:c734:9600:af27:4326:a216:2bfb]) by smtp.gmail.com with UTF8SMTPSA id ffacd0b85a97d-390e485ddd1sm15052695f8f.94.2025.03.03.08.30.54 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 03 Mar 2025 08:30:55 -0800 (PST) From: David Hildenbrand To: linux-kernel@vger.kernel.org Cc: linux-doc@vger.kernel.org, cgroups@vger.kernel.org, linux-mm@kvack.org, linux-fsdevel@vger.kernel.org, linux-api@vger.kernel.org, David Hildenbrand , Andrew Morton , "Matthew Wilcox (Oracle)" , Tejun Heo , Zefan Li , Johannes Weiner , =?utf-8?q?Michal_Koutn=C3=BD?= , Jonathan Corbet , Andy Lutomirski , Thomas Gleixner , Ingo Molnar , Borislav Petkov , Dave Hansen , Muchun Song , "Liam R. Howlett" , Lorenzo Stoakes , Vlastimil Babka , Jann Horn Subject: [PATCH v3 17/20] fs/proc/task_mmu: remove per-page mapcount dependency for PM_MMAP_EXCLUSIVE (CONFIG_NO_PAGE_MAPCOUNT) Date: Mon, 3 Mar 2025 17:30:10 +0100 Message-ID: <20250303163014.1128035-18-david@redhat.com> X-Mailer: git-send-email 2.48.1 In-Reply-To: <20250303163014.1128035-1-david@redhat.com> References: <20250303163014.1128035-1-david@redhat.com> Precedence: bulk X-Mailing-List: linux-fsdevel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Let's implement an alternative when per-page mapcounts in large folios are no longer maintained -- soon with CONFIG_NO_PAGE_MAPCOUNT. PM_MMAP_EXCLUSIVE will now be set if folio_likely_mapped_shared() is true -- when the folio is considered "mapped shared", including when it once was "mapped shared" but no longer is, as documented. This might result in and under-indication of "exclusively mapped", which is considered better than over-indicating it: under-estimating the USS (Unique Set Size) is better than over-estimating it. As an alternative, we could simply remove that flag with CONFIG_NO_PAGE_MAPCOUNT completely, but there might be value to it. So, let's keep it like that and document the behavior. Signed-off-by: David Hildenbrand --- Documentation/admin-guide/mm/pagemap.rst | 11 +++++++++++ fs/proc/task_mmu.c | 11 +++++++++-- 2 files changed, 20 insertions(+), 2 deletions(-) diff --git a/Documentation/admin-guide/mm/pagemap.rst b/Documentation/admin-guide/mm/pagemap.rst index d6647daca9122..afce291649dd6 100644 --- a/Documentation/admin-guide/mm/pagemap.rst +++ b/Documentation/admin-guide/mm/pagemap.rst @@ -38,6 +38,17 @@ There are four components to pagemap: precisely which pages are mapped (or in swap) and comparing mapped pages between processes. + Traditionally, bit 56 indicates that a page is mapped exactly once and bit + 56 is clear when a page is mapped multiple times, even when mapped in the + same process multiple times. In some kernel configurations, the semantics + for pages part of a larger allocation (e.g., THP) can differ: bit 56 is set + if all pages part of the corresponding large allocation are *certainly* + mapped in the same process, even if the page is mapped multiple times in that + process. Bit 56 is clear when any page page of the larger allocation + is *maybe* mapped in a different process. In some cases, a large allocation + might be treated as "maybe mapped by multiple processes" even though this + is no longer the case. + Efficient users of this interface will use ``/proc/pid/maps`` to determine which areas of memory are actually mapped and llseek to skip over unmapped regions. diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c index 1162f0e72df2e..f937c2df7b3f4 100644 --- a/fs/proc/task_mmu.c +++ b/fs/proc/task_mmu.c @@ -1652,6 +1652,13 @@ static int add_to_pagemap(pagemap_entry_t *pme, struct pagemapread *pm) return 0; } +static bool __folio_page_mapped_exclusively(struct folio *folio, struct page *page) +{ + if (IS_ENABLED(CONFIG_PAGE_MAPCOUNT)) + return folio_precise_page_mapcount(folio, page) == 1; + return !folio_maybe_mapped_shared(folio); +} + static int pagemap_pte_hole(unsigned long start, unsigned long end, __always_unused int depth, struct mm_walk *walk) { @@ -1742,7 +1749,7 @@ static pagemap_entry_t pte_to_pagemap_entry(struct pagemapread *pm, if (!folio_test_anon(folio)) flags |= PM_FILE; if ((flags & PM_PRESENT) && - folio_precise_page_mapcount(folio, page) == 1) + __folio_page_mapped_exclusively(folio, page)) flags |= PM_MMAP_EXCLUSIVE; } if (vma->vm_flags & VM_SOFTDIRTY) @@ -1817,7 +1824,7 @@ static int pagemap_pmd_range(pmd_t *pmdp, unsigned long addr, unsigned long end, pagemap_entry_t pme; if (folio && (flags & PM_PRESENT) && - folio_precise_page_mapcount(folio, page + idx) == 1) + __folio_page_mapped_exclusively(folio, page)) cur_flags |= PM_MMAP_EXCLUSIVE; pme = make_pme(frame, cur_flags);